This invention relates to vapor detection systems and particularly to methods and apparatus for temperature compensated vapor detection.
The electrical resistance sensitivity of organic semiconductor film to vapors is well known. Recently there has been considerable interest toward using the vapor sensitivity of organic semiconductors in micro-sensor applications where detection of a particular vapor is required (see U.S. Pat. No. 4,350,660 to Robinson et al.).
In a typical system, an organic semiconductor film is deposited onto an interdigitated surface conductivity cell, a potential is applied and the resulting current flow is measured. Variations in the current flow with changes in the vapor exposed to the device signal the presence of specific vapors or classes of vapors.
There are two serious problems encountered with this technique. First, the films have very high resistances even when used with the interdigitated electrodes. This necessitates the use of comparably high value resistors to provide a measurement reference whether used in a simple voltage divider, a wheatstone bridge or current-to-voltage converter circuit. Such high value resistors are physically large, expensive and somewhat unstable with time. The second problem is that the conductivity of the semiconductor film is highly temperature sensitive. Thus, it is very difficult to discriminate between signal changes caused by small temperature variations and those caused by small vapor concentration changes. Yet a third problem is that the aforesaid techniques requiring high value reference resistors are not suitable for the microfabrication which is necessary for mass production of economical and reliable devices.
The prior art has dealt with the problem of temperature compensation a number of different ways.
U.S. Pat. No. 4,112,356 to Toy discloses a semiconductor vapor detector circuit having automatic temperature compensation which relies upon the principle of swamping out the temperature coefficient of one critical element with the temperature coefficient of another component having opposite polarity. In this case, the semiconductor sensor has a negative coefficient. An operational amplifier for amplifying the detector output is selected having a positive temperature coefficient. A portion of the output signal derived by resistance voltage division, along with a resistance generated voltage proportional to amplifier supply current, is negatively fed back to the amplifier input. The negative feedback generated by returning a certain amount of signal proportional to both amplifier output and supply current can be adjusted by resistance voltage division so that the temperature coefficient effects of the amplifier should cancel the temperature coefficient of the semiconductor vapor sensor. However, this system has some serious shortcomings. First, a combination of resistance voltage dividers must be used to properly balance the temperature coefficients of the sensor and amplifier, increasing complexity and making the apparatus more difficult to fabricate, especially if it is to be fabricated on an integrated circuit. In addition, unless the components for the system are manufactured to extremely narrow tolerances, the resistance voltage division circuitry will have to be adjustable. This adds to cost and complexity, and prohibits microfabrication as well. Finally, the operational amplifier must operate under the same ambient conditions as the sensor itself. This limitation can increase cost and fabrication problems for some applications.
U.S. Pat. No. 4,361,802 to Luippers discloses another system of temperature compensation using resistance wires for vapor analysis. A wire element is used in each leg of a bridge circuit, two of the elements in opposite legs of the bridge being exposed to the vapor to be measured, and the remaining two elements being exposed to a reference vapor. The difference in thermal conductivity between the sampled vapor and the reference causes an imbalance in the bridge and a corresponding voltage developed on its output. To ensure that the reference elements retain the same resistance under varying thermal conditions, the current to the bridge is controlled by connecting this primary bridge in one leg of a secondary bridge circuit, using the resistance of the primary bridge to control a current regulator circuit to keep the current fed to the primary bridge constant, thereby providing temperature compensation for any thermally induced resistance shift. This system could be adapted for semiconductor vapor sensors, but it has several disadvantages. First, it requires the use of four sensor elements, two of which must be exposed to a reference gas. It also requires the use of a secondary bridge circuit and associated current control circuitry, which requires the use of thermally stable high impedance resistances, increasing cost and making microfabrication difficult. Finally, unless all components are held to extremely tight tolerances, the secondary bridge must be adjusted and aligned to provide the proper degree of compensation. This reduces economy and reliability of the apparatus and makes microfabrication of the entire circuit impossible
U.S. Pat. No. 4,217,544 to Schmidt discloses an automatic temperature compensation system for a corrosion measuring system. This system uses a reference sensor thermally coupled to the active sensor. The reference sensor and associated circuitry generates a signal that is proportional to temperature. This temperature signal is then subtracted from the uncompensated output signal from the active sensor circuitry to produce a temperature compensated output. This system has significant problems if adapted for temperature compensation using semiconductor vapor sensors. First, both the active and reference sensors must have preamplification, requiring a multiplicity of operational amplifiers and associated high impedance resistances. This increases cost and complexity, and if the circuit is to be microfabricated onto a single integrated circuit, the amplifiers and resistances must PG,7 have a high degree of thermal stability as well, driving up cost still further. In addition, the reference temperature signal and the uncompensated active sensor signal must be balanced in the subtractor circuit for the proper degree of temperature compensation, making entire fabrication of the circuit onto single integrated circuit impossible and requiring the use of external calibration controls, thereby increasing cost and complexity.